fix(api): pause memory uploads

[daniellam258]

Summary

• pause all four client-facing memory write endpoints at the routing boundary
• return HTTP 404 with the security-upgrade message in the standard JSON error shape
• keep recall, status polling, and restore endpoints available

Why

New Walrus Memory uploads need to be suspended while a security upgrade is conducted. Routing write requests directly to the pause handler prevents validation, extraction, database inserts, and background upload jobs from starting.

Impact

The following endpoints now return 404 with {"error":"New uploads to Walrus Memory are paused while we conduct a security upgrade"}:

• POST /api/remember
• POST /api/remember/bulk
• POST /api/remember/manual
• POST /api/analyze

Validation

• cargo fmt --manifest-path services/server/Cargo.toml -- --check
• cargo test --manifest-path services/server/Cargo.toml uploads_paused_tests
• git diff --check